J. MATER. CHEM., 1994, 4(4), 551-555 551

Intercalation of Organic Compounds in the Layered Host Lattice MOO,

Hideyuki Tagaya,* Kensuke Ara, Jun-ichi Kadokawa, Masa Karasu and Koji Chiba Department of Materials Science and Engineering, Yamagata University, Yonezawa, Yamagata 992, Japan

Organic compounds such as alkylpyridines, methyl viologen and azo compounds have been intercalated into MOO, layers by ion exchange in which MoO",-(Na+), reacted with the organic compounds. lndolinespirobenzopyran (SP) was intercalated by a multi-step method in which amine was intercalated in the first step, and by the co-intercalation of amine and SP the MOO, layer expanded to 24-28A depending upon the kind of amine used in the first step. In the layers, SP existed as its isomerized coloured species, merocyanine (MC). Reduction of Mo by chemical reduction and intercalation of amines and SP was confirmed by X-ray photoelectron spectroscopy.

Many layered solids act as host lattices and react with a variety of guest molecules to give intercalation com- pounds in which the guest is inserted between the host layers.'-" They are candidates for electronic devices and heterogeneous catalysts. Among host materials, oxides are expected to form stable intercalation compounds. Layered MOO, is such a host.

The binary molybdenum(vi) oxide MOO, can be described in terms of a typical layer structure in which distorted MOO, octahedra share edges and vertices to form two-dimensional (MOO,) sheets which are separated by a van der Waals gap (Scheme l).4 It is well known that insertion of hydrogen into the layers produces molybdenum bronzes, HxMo035-7 and MOO, can take up alkali metals such as the Li+ ion thermally,* c h e m i ~ a l l y ~ , ~ ~ and electrochemically.11.'2 Sodium ions are also chemically inserted easily into the layers by using Na,S,O, to form the hydrated sodium molybdenum oxide, MOO$- (Na'),, as shown in Scheme l . 1 3 Schollhorn et al. reported that H,MoO, and MoO$-(Na+), reacted with organic bases to give organic intercalation corn pound^.'^^^^

We have been studying the formation of new intercalation compounds in insulator hosts1G18 and hosts with electronic condu~t iv i ty .~~ In the latter case, reduction or oxidation of the host component should occur. X-Ray photoelectron spec- troscopy (XPS) is a useful means to observe the valence state of such hosts; however, few studies have been reported on the valence state of MOO, in the course of its intercalation reaction. In this work we present the results of a study on the formation and characteristics of organic intercalation com- pounds including pyridines, azocompounds and indolinespiro- benzopyran (SP) as guests in MOO,.

Experimental Intercalation

Organic solvents were of analytical grade. They were dried and fractionated prior to use, if necessary. All other materials used were from commercial suppliers and used as received.

Chemical reduction of MOO, and the cation-exchange reactions were carried out in the following manner. MOO, (0.1 g) was suspended in an aqueous solution of Na,S,O, ( 1 mol 1-I; 1 ml) at room temperature for 1 h. The colour of the solid changed from white to dark metallic blue, indicating the formation of MoO$-(Na+),. The solvent was decanted and the residual wet solid was treated with an ethanolic solution of an excess of guest at 80 "C for 24 h. After reaction, the products were filtered and washed with ethanol.

Direct thermal intercalation of amines and aniline (> three times the concentration of MOO,) into the MOO, host (0.2-1.0 g) were carried out at 80°C for 2-24 h under a dry nitrogen atmosphere. Reaction products were filtered and washed with ace tone.

In the case of the multi-step intercalation, the indercalate obtained (0.05-0.1 g) was allowed to react with SP (0.3 g) in toluene at 80 "C for 24 h.

Characterization of the Intercalates

Powder X-ray diffraction (XRD) spectra were recorded on a Rigaku powder diffractometer unit using Cu-Ka ( filtered) radiation at 40 kV and 20 mA. XP spectra were collected on a Shimazu ESCA using a monochromatic Mg-Kr X-ray source. XP spectra of the intercalation compounds were taken

T 6.93 A

Na2S2°4 - T ca. 11.4 A

Scheme 1 Insertion of Na+ into MOO,

Publ

ishe

d on

01

Janu

ary

1994

. Dow

nloa

ded

by M

cMas

ter

Uni

vers

ity o

n 10

/12/

2013

08:

25:2

9.

View Article Online / Journal Homepage / Table of Contents for this issue

552 J . MATER. CHEM., 1994, VOL. 4

after the samples had been etched by an argon-ion beam. Thermal analyses (TG/DTA) were performed on a Seiko SSC5000 thermal analysis system using a heating rate of 10 "C min-'.

Results and Discussion Intercalation by Ion Exchange

Intercalation of non-donor organics into MOO, was fairly difficult since the first electron-transfer process from the guest to MOO, was believed to be the rate-determining step. That is, the organic guest should be a donor. Schollhorn reported that the kinetics of the intercalation process of MOO, depended upon the basicity of the Lewis base.

By thermal reaction of the organics with MOO,, low pK, compounds such as tetradecylamine (pKb = 3.13) were interca- lated relatively easily; however, reaction of pyridine ( pKb = 8.75) with MOO, was fairly slow (reported conditions were 180 "C for 30 days). To obtain new intercalation compounds, the Na' intercalation compound was treated with high pKb organics.

By reaction of MOO, with an aqueous Na,S,O, solution, the MOO, layer expanded from 6.9 to 11.4 A, indicating that the hydrated Na' ion was intercalated as s h o p in Scheme 1. The value of the layer expansion wasoca. 4.5 A. As the Stokes ionic radius of hydrated Na' is 3.3 A:' the layer expansion was reasonable. However, thisocompound was not stable and the lowest-angle peak (11.4A) disappeared. As shown in Fig. 1 (b), the interlFyer spacing of a dry sample under reduced pressure was 9.74. This decrease in the interlayer spacing from 11.4 to 9.7 A indicated that partial dehydration had occurred. Therefore, we used wet MoO'j-'(Na+), as the reac- tant with organic guests.

By reaction of 4-propylpyridine with wet MoOz-(Na f)x,

the interlayer spacing increased to 12.2 A; however, the amount of intercalated 4-propylpyridine confirmed by thermal analysis was small and it was not single phase. This showed

2 O/de g lees

Fig. 1 XRD patterns of (a) MOO,, (b) MoO;-(Na+), and (c) 4-aminoazobenzene intercalate

that the exchange reaction of Na' with alkylpyridine was not easy. Therefore, 4-propylpyridine was treated with hydro- chloric acid to yield 4-propylpyridinium hydrochloride which reacted with the wet MoO$-(Na+), in ethanol. The product was an orthorhombic single phase, as shown in Table 1. The analysed composition of the intercalate was MOO, (4-propylpyridine),.

Thermal analysis of 4-propylpyridine and the reaction product are shown in Fig. 2. Although all of the 4- propylpyridine was lost below 200 "C, the weight loss began at 350°C in the case of the reaction product and continued even to 600°C. The pKb of aniline is 9.42, higher than those of alkylpyridines, as shown in Table 2. Aniline was also intercalated by reaction of wet MoO:-(Na+), with the anilin- ium hydrochloride. The pKbs of 4-aminopyridine and 4,4'- azodianiline were higher than those of alk ylpyridines and

Table 1 XRD data for MOO, (C,H,C5H,NHi)o,13

2Oldegrees I / I o h k 1 dobs/A dead 6.74

13.60 21.16 22.98 25.96 27.56 31.60 33.40 34.76 45.32 49.34

100 0 2 0 1 3 0 4 0 1 3 0 6 0 2 8 1 1 0 2 2 1 4 0 2 3 0 4 1 2 1 0 6 1 1 1 1 0 1 2 0 1 3 0 2 0 2 0 0 1 1 0 0 2

13.10 6.5 1 4.33 3.87 3.43 3.23 2.83 2.68 2.58 2.00 1.85

13.08 6.54 4.36 3.91 3.38 3.22 2.82 2.70 2.58 1.97 1.85

I I I # 100 200 300 400 500 600 J TI"C

Fig. 2 Thermogravimetric curves for (a) MOO,, ( h ) methyl viologen intercalate, (c) methyl viologen, ( d ) 4-propylpyridine intercalate and (e) 4-propylpyridine

Table 2 Lattice parameters for intercalation compounds, MOO, (organic guest),

lattice paramet er/A ~

pKb a b c d / A guest (x)

decylamine 3.13 3.96 56.23 3.69 28.1 4-propylpyridine (0.13) 7.86 3.95 26.15 3.70 13.1 4-methylpyridine 7.92 3.98 26.32 3.70 13.2 aniline (0.04) 9.42 3.97 27.82 3.70 13.9 4,4'-azodianiline 10.76, 11.36 3.93 26.80 3.75 13.4 4-aminoazobenzene (0.08) 11.06 3.97 26.61 3.73 13.3 methylviologen (0.23) 3.98 24.51 3.70 12.3

Standard deviation < 0.2 except for 0.4 for decylamine intercalate.

Publ

ishe

d on

01

Janu

ary

1994

. Dow

nloa

ded

by M

cMas

ter

Uni

vers

ity o

n 10

/12/

2013

08:

25:2

9.

View Article Online

J. MATER. CHEM., 1994, VOL. 4 553

aniline, and their intercalates in MOO, have not yet been reported. They were also treated with hydrochloric acid and wet MoO;-(Na+),. They (and methyl viologen) were intercalated by this ion-exchangeornethod and the interlayer spacings expanded to 12.3-13.!A as shown in Table2. The layer expansions were 5.4-6.9 A. As an example, the XRD pattern of the 4-aminoazobenzene intercalate is shown in Fig. 1 (c). The layer expansions indicate that the guests were hydrated; the planes of the guests were parallel to the host layers.

Methyl viologen has received considerable attention as being an efficient electron relay in the photocatalysed reduction of water to hydrogen.21*22 The thermal analysis of methyl viologen and its intercalate are shown in Fig. 2(b) and (c). The weight loss of methyl viologen intercalate continued to 600'C, indicating that methyl viologen in the layers was more stable thermally than methyl viologen itself.

Intercalation by Multi-step Reactions

Indolinespirobenzopyran (SP) is a well known photochromic (Scheme 2) for which the coloured species is

merocyanine (MC). SP was not intercalated by direct thermal reaction with MOO, because SP is not a donor. Alkylamine donors with pK,s < 4 were used as the guests for the hosts of the various layer compounds. We previously confirmed the usefulness of multi-step intercalation." In this study, hexylam- ine, decylamine and tetradecylamine reacted with MOO, thermally, and the intercalates obtained reacted with SP. In many hosts alkylamines are known to be intercalated within a short time; however, the reactions of alkylamines with MOO, were not faster and in some cases it required reaction times of up to 24 h for completion. In the thermal analysis decylamine was lost below 180 "C, as shown in Fig. 3; however, the loss of decylamine intercalate occurred above 400 "C. For MoO,(decylamine), x was calculated to be 1.57 from the weight decrease in which we assumed that all of the interca- lated decylamine was evolved by thermal treatment up to 600°C. By thermal analysis of the reaction products of the decylamine intercalate with SP, the weight loss began above 200 "C and continued to 600 "C, confirming the intercalation of SP into MOO,. The composition of the intercalate was calculated to be MoO,(decylamine),~, (SP),,,,, assuming that decylamine was evolved from 100 to 265°C and SP was evolved from 265 to 600°C. Decylamine might be evolved even above 265°C. Therefore, the calculated value of the

100

- 80 8 v

$

60

40

100 200 300 400 500 600 T/"C

Fig. 3 Thermogravimetric curves for (a) decylamine and SP co-intercalate, (b) decylamine intercalate and (c) decylamine

intercalated SP was the maximum value. The XRD spectrum showFd that the interlayer spacings decreased from 37.7 to 28.7 A in the reaction of the tetradecylamine intercalate with SP (Fig. 4). The products were not a single phase and include the host MOO, itself. This suggests the occurrence of deinter- calation of the amine, probably because SP is not a donor. Therefore, the interlayer spacings of the products were calcu- lated only from the low-angle peaks that do not correspond to the peaks of MOO, itself. Table 3 shows the change in the interlayer spacing in the course of each step. At the h t step, the intercalatdon of amines, the interlayer spacing expanded to 18.7-37.8 A depending on the length of aliphatic moieties. From the values of the layer expansions and the length of amines, the formation of tilted bilayers26 of amines was suggested. At the second step, the interc?lation of SP, the interlayer spacing decreased to 24.2-28.7 A. The decrease in interlayer spacing suggested deintercalation of some amines. The maximum in the fluorescence spectra of the SP intercalate was near 600nm, indicating that SP exits in its isomerized form, MC, in the layers. From these results, two reaction mechanisms were considered. The first mechanism was that

10 20 30 40 50 2 $/degrees

Fig. 4 X R D patterns for (a) tetradecylamine, (b) tetradecylamine intercalate and (c) tetradecylamine and SP co-intercalate

Table 3 Interlayer spacings, d, for intercalation compounds of MOO, with amines and spiropyran

guest lattice parameter/A

first step second step a b c d/h Ad/A

hex ylamine - 3.96 37.35 3.70 18.7 11.8 dec ylamine - 3.96 56.23 3.69 28.1 21.2 decylamine spiropyran 3.73 48.31 3.87 24.2 17.3 tetradecylamine - 4.07 75.55 3.65 37.8 30.9 tetradecylamine spiropyran 3.96 57.34 3.69 28.7 21.8

Standard deviation < 0.6.

Publ

ishe

d on

01

Janu

ary

1994

. Dow

nloa

ded

by M

cMas

ter

Uni

vers

ity o

n 10

/12/

2013

08:

25:2

9.

View Article Online

554 J. MATER. CHEM., 1994, VOL. 4

SP reacted with anionic species in the layers directly to give MC. The other mechanism was that SP was co-intercalated by hydrophobic-hydrophobic interaction with alkylamines in the layers, and then SP reacted with host layers to give MC.

XPS Spectra of the Intercalates

The XPS spectra of MOO, and MoO';-(Na'), are shown in Fig. 5. Peaks of non-intercalated Mo 3d3,2 and Mo 3d,,, occur at 238.2 and 234.9 eV, respectively. It is well known that the peaks of Mo 3d are shifted to higher energy by reduction of the The peaks of MoXV in MoS, are at 233.6 and 230.8eV, as shown in Table4. On intercalation of the Na' ion, the peaks are shifted to higher energies, 234.4 and 231.4eV. They are close to the values for MoS,, indicating the reduction of the Mo metal by intercalation. Mo was partially reduced by the chemical reduction of MOO, host. Therefore, there should be at least two molybdenum states. The results, in which only one peak was observed, indicated that the electrons involved in the reduction of Mo were not localized.

Peaks in the XPS spectrum of the 4-propylpyridine interca- late were at 235.3 and 232.4 eV (Fig. 6) and were shifted slightly to low energies from those of MoO'j-(Na+), (Table 4). This shows that the change in the valence state of Na+- intercalated MOO, in the course of intercalation of

240 235 230 225 220 EdeV

4-propylpyridine was small. When the sample was oxidised using H202, the values shifted to lower energies, in agreement with the values of non-intercalated MOO, (Fig. 6). The XRD peaks of the intercalate disappeared after oxidation of the sample, indicating that oxidation resulted in deintercalation and destruction of the layer structure. In the case of aniline, aminoazobenzene and methyl viologen, the peaks shifted to lower energies during the course of intercalation; however, the values were higher than that of MOO, itself. In the case of aniline the values were shifted to lower values by oxidation of the sample. These facts indicate that intercalation of organic salts by ion-exchange occurred while the reduction state of the Mo metal was retained.

When neutral 4-propylpyridine was intercalated, the XPS peaks were close to those of non-intercalated MOO, (Table 4). This shows that in the course of such an intercalation reaction, Na' was deintercalated accompanied by a small adsorption of 4-propylpyridine, as already mentioned. By the multi-step intercalation of SP, Mo was reduced, as shown in Fig. 7. In the case of multi-step reactions of SP, the deintercalation of amines was observed and the XRD peaks of MOO, itself were observed. However, the peaks of Mo shifted to higher energies, close to those of MoS,. MC is an ionized species, as shown in Scheme2, and has an ammonium ion. When the sample was irradiated with UV light, isomerization of MC to colour- less SP did not occur, indicating that MC was stable in the layers. It is possible that MC was stabilized by ion-ion interaction in the layers. The reduction state of Mo was also stable as a result of the interaction with the ammonium ion.

Table 4 Summary of XPS binding energies of Mo 3d

MOO, none MoS, none MOO, Na+ MOO, 4-methylpyridine MOO, 4-propylpyridine MOO, 4-propylpyridine-o" MOO, 4-propylpyridine-nb MOO, methyl viologen MOO, 4-aminoazobenzene MOO, 4-aminoazobenzene. MOO, spiropyran MOO, aniline MOO, aniline-o'

238.2 233.6 234.4 236.2 235.3 238.4 238.2 237.5 237.2

-nb 237.3 233.6 237.5 238.3

234.9 230.8 231.4 233.2 232.4 235.0 235.3 234.3 234.4 233.7 231.2 234.4 235.2

Fig. 5 Mo 3d XPS spectrum of MOO, (---) and MoO:-(Na+), (-) 'Intercalate oxidized using H,O,. bIntercalate obtained by the reaction of MOO, with neutral guest.

240 235 230 225 220 EdeV

I I A

240 235 230 225 2:

Fig. 6 Mo 3d XPS spectrum of 4-propylpyridine intercalate (-) and oxidation product of 4-propylpyridine intercalate (- -- )

EdeV

Fig. 7 Mo 3d XPS spectrum of decylamine and SP co-intercalate

10

Publ

ishe

d on

01

Janu

ary

1994

. Dow

nloa

ded

by M

cMas

ter

Uni

vers

ity o

n 10

/12/

2013

08:

25:2

9.

View Article Online

J. MATER. CHEM., 1994, VOL. 4 555

-+- NO?

Spiropyran (SP)

CH,

Merocyanine (MC)

Scheme 2

Conclusions Weak bases having high pKb values, such as azo compounds, were intercalated in the layer of MOO, by the ion-exchange method. SP was also intercalated by the multi-step method in which the amine intercalate reacted with SP. The reduction state of Mo was retained in the course of the intercalation reactions, indicating that the driving force of intercalation is ion-ion interactions. The intercalation process leads to the preparation of new classes of materials. The establishment of the ion-exchange and the multi-step methods contributes to the preparation of organic-inorganic nanocomposites.

The authors are grateful to Mr. T. Murayama and S. Sat0 for their technical collaboration.

References

1 W. Y. Liang, in Intercalation in Layered Materials, ed. M. S. Dresselhause, Plenum Press, New York, 1986, p. 31.

2 R. Schollhorn, in fnclusion Compounds, ed. J. L. Atwood, J. E. D. Davies and D. D. MacNicol, Academic Press, New York, 1984. vol. 1, p. 249. A. J. Jacobson and S. Whittingham, Intercalation Compounds, Academic Press, New York, 1982. G. Andersson and A. Magneli, Actu Chim. Scund., 1950,4,793. P. G. Dickens, S. Crouch-Baker and M. T. Weller, Solid State fonics, 1986, 18&19,89.

3

4 5

6 7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23 24

25

26

27

K. Eda, J. Muter. Clzem., 1992, 2, 533. S. Crouch-Baker and P. G. Dickens, Muter. Res. Bull., 1984, 19, 1457. W. H. McCarroll and M. Greenblatt, J . Solid Stare C h t m , 1984, 54,282. S. J. Hibble, P. G. Dickens and J. C. Evison, J. Chem. Sclc., Chem. Comrrzun., 1985, 1809. P. G. Bruce, F. Krok, J. Nowinski, V. G. Gibson and K. Tavakkoli, J. Mater. Chem., 1991,1, 705. J. 0. Besenhard, J. Heydecke, E. Wudy and H. P. Fritz, Sdid State Ionics, 1983,8,61. M. Pasquali, F. Rodante and G. Pistoia, Thermochim. AL-ta, 1987, 121, 203. T. Iwamoto, Y. Itoh, K. Ohkawa and M. Takahashi. Nippon Kagaku Kuishi, 1983,273. R. S. Schollhorn, R. Kuhlmann and J. 0. Besenhard, MLiter. Res. Bull., 1976, 11, 83. R. Scholhorn, T. Schulte-Nolle and G. Steinhoff, J . Less- Common Metals, 1980,71, 71. H. Tagaya, T. Kuwahara, S. Sato, J. Kadokawa, M. Karasu and K. Chiba, J. Muter. Chem., 1993,3, 317. H. Tagaya, K. Saito, T. Kuwahara, J. Kadokawa and k. Chiba, Catal. Today, 1993, 16,463. H. Tagaya, S. Sato, H. Morioka, J. Kadokawa, M. Karasu and K . Chiba, Chem. Muter., 1993,5, 1431. H. Tagaya, T. Hashimoto, M. Karasu, T. Izumi and K. Chiba, Chem. Lett., 1991, 2113. R. Fujishiro, G. Wada and R. Tamamushi, in Yoeki no Seishitu, Tokyo Kagaku Dojin, Tokyo, 1968, p. 61. A. Harriman, G. Porter and M. Richoux, J. Chem. Soc., Faraday Trans. 2, 1981,77, 1175. H. Tagaya, H. Saito, S. Suda and K. Chiba, Bull. Chem. Soc. Jpn., 1989, 62,768. H. Durr, Angew. Chem., f n t . Ed. Engl., 1989,413. K. Takagi, T. Kurematsu and Y. Sawaki. J . Chem. S O C , Perkin Truns 2,1991,1517. J. Z. Zhang, B. J. Schwartz, J. C. King and C. B. Harris, J. Am. Chem. Soc., 1992,114,10921. J. F. Lambert, Z . Deng, J. B. D'Espinose and J. J. Fripiat, J. Colloid Interface Sci., 1989, 132,337. R. Holm and S. Storp, Appl. Phys., 1976,9,217.

Paper 3/04458K; Receiued 27th July, 1993

Publ

ishe

d on

01

Janu

ary

1994

. Dow

nloa

ded

by M

cMas

ter

Uni

vers

ity o

n 10

/12/

2013

08:

25:2

9.

View Article Online